1. Field of the Invention
This invention relates to a method of preparing compositions of homopolymers and copolymers of limited molecular weight, referred to as macromonomers, which contain an olefinic polymerizable end group wherein the monomeric units comprise methacrylate, methacrylonitrile, methacrylamide, derivatives thereof, and other monomers and mixtures thereof.
2. Background
Macromonomers, that is, polymers containing an olefinic polymerizable end group, are known in the art. For example, U.S. Pat. No. 5,028,677 discloses a process of obtaining compositions containing a higher percentage of macromonomer in order to permit more effective utilization of these compounds in the production of graft polymers and other polymeric moieties.
In producing macromonomers, as to some extent in producing polymers in general, it is necessary to be able to control the molecular weight of the product of polymerization so that it may be fitted to its particular use or need. Unperturbed polymerization systems fundamentally tend to produce high molecular weight polymers, whereas it may be desirable or necessary, as in the case when producing macromonomers, to limit the molecular weight of the polymerization product. This must be done in a fairly predictable and controllable fashion.
For free radical polymerization processes, there are a number of conventional means of effecting such molecular weight limitation. These, along with notable disadvantages or problems, include
(1) A high initiator/monomer ratio. However, this may be costly in terms of initiator consumption. Also, high initiator levels may also produce undesirable end groups on the polymers produced. PA1 (2) Polymerization at high temperatures. However, this may lead to undesirable depropagation, thermal initiation, and undesirable secondary reactions. PA1 (3) Adding stoichiometric amounts of thiol chain transfer agents to the polymerizing system. However, the attendant incorporation of sulfur-containing agents into the polymer may render it less durable than is desired. There may also be odor problems associated with the use of sulfur-containing chain transfer agents. PA1 (4) Chain transfer agents employing cobalt (II or III) chelates such as disclosed in U.S. Pat. No. 4,680,352, U.S. Pat. No. 4,694,054, and WO 87/03605 published Jun. 18, 1987. However, a possible disadvantage of these agents is that some are adversely affected or deactivated by low pH. Also, they may tend to cause color problems, especially if interaction with some monomers may require higher levels of the cobalt chelate. PA1 (5) Group transfer agents such as disclosed in U.S. Pat. Nos. 4,417,034 and 4,414,372 or European patent application publication 0 248 596. However, they are specific to methacrylated polymers and relatively expensive. They are also sensitive to certain monomers and impurities which contain active hydrogens, e.g., hydroxyl, carboxyl, carboxylic acid, or amine. Special solvents may be required or blocked hydroxyl and carboxyl monomers may be needed which require a special de-blocking step to activate the protected group.
The use of terminally or .omega.-ethylenically unsaturated oligomers as chain transfer agents, for controlling the molecular weight of certain polymers in some contexts, is also known. There have been a number of studies and articles on the chain transfer properties of such oligomers.
Such oligomers are known, for example, as disclosed in U.S. Pat. No. 4,547,327; U.S. Pat. No. 4,170,582; U.S. Pat. No. 4,808,656, Japanese patent 3,161,562; Japanese patent 3,161,593. See also, P. Cacioli, et al., J. Makromol. Sci.-Chem., A23 (7), 839-852 (1986) and H. Tanaka, et al., Journal of Polymer Science; Part A; Polymer Chemistry, 27, 1741-1748 (1989).
It is an object of this invention to provide a method of polymerization to obtain macromonomers by employing a .omega.-unsaturated oligomer as a chain transfer agent.
It is a further object to control the molecular weight of the macromonomer so produced.
The present invention avoids problems associated with chain transfer agents which have been previously in commercial use, for example in the production of coatings and finishes. The present method has a number of significant advantages, including lower polymerization temperatures, reduced initiator costs, and less color. In general, the present method thus has the advantage of reducing the variables that narrow the utility of commonly used chain transfer agents or techniques that are commonly used.
These and other objects will become apparent hereinafter.